Method for producing hydronycarboxylic acid esters

ABSTRACT

The invention relates to mixtures of mono-, di-, and tri-esters of hydroxycarboxylic acids and to a process for producing them. The hydroxycarboxylic acid is preferably citric acid. In the disclosed process, hydroxycarboxylic acids or salts thereof are reacted with a mixture of alcohols corresponding to general formulae (I) R 1 —OH and (II) R 2 —(C 2 H 4 ) n —OH, where R 1  and R 2  independently of one another represent a saturated or unsaturated, branched or unbranched C 6-22  alkyl group and n is a number of 1 to 20, at temperatures of 120 to 180° C., wherein the compounds of formulae (I) and (II) are used in a ratio by weight of 10:1 to 1:10, and preferably in a ratio of 10:1 to 1:1.

This invention relates to a process for the production of selectedhydroxycarboxylic acid esters and mixtures of hydroxycarboxylic acidesters and to their use in cosmetic compositions.

Hydroxycarboxylic acids are well-known compounds.Monoalkylhydroxycarboxylic acid esters in particular have long been usedin cosmetic preparations. Thus, EP 282 289 A1, for example, describes acosmetic composition containing monoalkylcitric acid salts. Besides thepure alkyl compounds, alkyl oxide compounds of citric acid are alsodisclosed in that document.

WO 94/10970 describes a solubilizer containing monoalkyl citrates withC₇₋₁₀ alkyl groups as an ingredient of perfumes, cosmetic compositions,such as body cleansing and care preparations, and textiles. DE 199 45478 A1 describes cosmetic and/or pharmaceutical preparations which,besides alkyl and/or alkenyl oligoglycosides, contain hydroxycarboxylicacid partial esters or salts thereof.

Unfortunately, the known products are attended by various disadvantages.Thus, the known alkyl citrates often cannot be made up into clearformulations in conjunction with anionic surfactants and, even when theyare combined with certain nonionic surfactants, such as alkylpolyglycosides, formulation problems can still arise. In addition, thepure alkyl citrates are present as high-melting pastes at roomtemperature. Accordingly, the problem addressed by the present inventionwas to overcome the disadvantages of known alkyl citrates andhydroxycarboxylic acid esters. It has been found that alkyl citrateswithout any of the disadvantages mentioned above can be obtained throughthe choice of the alcohol component.

The present invention also relates to a process for the production ofhydroxycarboxylic acid esters, in which hydroxycarboxylic acids orhydroxycarboxylic acid salts are reacted with a mixture of alcoholscorresponding to general formula (I) R¹—OH and (II) R²—(C₂H₄)_(n)—OH,where R¹ and R² independently of one another represent a saturated orunsaturated, branched or unbranched C₆₋₂₂ alkyl group and n is a numberof 1 to 20, at temperatures of 120 to 180° C., characterized in that thecompounds of formulae (I) and (II) are used in a ratio by weight of 10:1to 1:10. The present invention also relates to the compounds obtainedwhich are mixtures of various isomeric esters.

In principle, the process according to the invention may be carried outusing any hydroxycarboxylic acids, particularly preferredhydroxycarboxylic acids being selected from the group consisting oflactic acid, tartaric acid, malic acid and citric acid andself-condensation products thereof. Citric acid is particularlypreferred for the purposes of the invention.

The partial esters of hydroxy carboxylic acids in the context of theinvention are surfactants which, preferably, still contain a freecarboxyl group. Accordingly, the partial esters may also be acidicesters or neutralization products thereof. In that case, the partialesters are present in the form of alkali metal, alkaline earth metal,ammonium, alkylammonium, alkanolammonium and/or glucammonium salts.

The esters themselves are preferably derived from fatty alcohols offormula (I) which are used in admixture with ethoxylated fatty alcoholscorresponding to general formula (II). According to the invention, theratio by weight between the alcohols of formula (I) and the ethoxylatedalcohols of formula (II) must be in the range from 10:1 to 1:10. In aparticularly preferred embodiment, the alcohols of formula (I) and (II)are used in a ratio by weight of 10:1 to 1:1, more particularly 9:1 to1:1, preferably 4:1 to 1:1 and most particularly 1:1.

The esters according to the invention are polyesters because severalcarboxyl functions can be esterified. Typically, the esters are in theform of mixtures from their production, of which about 25 to 30% may beformed by monoesters, 30 to 40% by diesters and 5 to 15% by triesters.The balance to 100% is formed by free hydroxycarboxylic acid.

Accordingly, the present invention also relates to mixtures of isomericcompounds corresponding to general formula (III):

in which R′, R″, R′″ represent a hydrogen atom and/or a C₆₋₂₂ alkylgroup and/or an ethoxylated C₆₋₂₂ alkyl group, the ethoxylated alkylgroups containing 2 to 20 parts ethylene oxide per alkyl group, with theprovisos that at least one of the substituents R′, R″ and R′″ representssuch an ethoxylated alkyl group and the total number of ethylene oxideunits per ester molecule is limited to 20. The mixtures contain mono-,di- and triesters alongside one another, mono- and diesters preferablybeing present in a ratio of 1:1 to 1:2. The percentage content of freecitric acid may be up to 20%, based on the mixtures. However, themixtures preferably contain less free citric acid, preferably less than10%.

The production process as such corresponds to the prior art. It can beessential to carry out the reaction in a nitrogen atmosphere. Inaddition, it can be of advantage to carry out the reaction attemperatures of 150 to 170° C. and preferably at 160° C. Themonoalkylesters of the hydroxycarboxylic acids according to theinvention are obtained as the end product and may be removed from themixture, for example by distillation. The esters may be present in freeform or as salts. A small percentage of the hydroxycarboxylic acid,preferably at most 20% by weight and more particularly at most 10% byweight, remains unesterified in the process. Reaction productscontaining at most 8% and more particularly at most 5% unesterifiedcitric acid are particularly preferred.

The acid value (DIN 51963) of the products obtained in accordance withthe invention is preferably in the range from 200 to 300 while their OHvalue is preferably in the range from 180 to 250, their ester value ispreferably in the range from 100 to 160 and their saponification valueis preferably in the range from 380 to 500 (all measurements to DIN).

The hydroxycarboxylic acid esters produced in accordance with theinvention and preferably the esters of citric acid may advantageously beformulated with anionic and/or nonionic surfactants to form aqueoussolutions.

Particularly preferred nonionic surfactants are inter alia fattyalcohols, alcohol ethoxylates and alkyl polyglycosides. Particularlysuitable anionic surfactants are alkyl ether sulfates although thechoice of the anionic surfactants is not limited to alkyl ethersulfates.

Fatty Alcohols

The fatty alcohols which are also used in the synthesis of thehydroxycarboxylic acids according to the invention correspond to formula(I):R¹OH  (I)where R¹ is an aliphatic, linear or branched hydrocarbon radicalcontaining 6 to 22 carbon atoms and 0 and/or 1, 2 or 3 double bonds.Typical examples are caproic alcohol, caprylic alcohol, 2-ethylhexylalcohol, capric alcohol, lauryl alcohol, isotridecyl alcohol, myristylalcohol, cetyl alcohol, palmitoleyl alcohol, stearyl alcohol, isostearylalcohol, oleyl alcohol, elaidyl alcohol, petroselinyl alcohol, linolylalcohol, linolenyl alcohol, elaeostearyl alcohol, arachyl alcohol,gadoleyl alcohol, behenyl alcohol, erucyl alcohol and brassidyl alcoholand the technical mixtures thereof obtained, for example, in thehigh-pressure hydrogenation of technical methyl esters based on fats andoils or aldehydes from Roelen's oxo synthesis and as monomer fraction inthe dimerization of unsaturated fatty alcohols. Preferred fatty alcoholsare technical C₁₂₋₁₈ fatty alcohols such as, for example, coconut oil,palm oi, palm kernel oil or tallow fatty alcohol.Alcohol Ethoxylates

Alcohol ethoxylates are known as fatty alcohol or oxoalcohol ethoxylatesfrom their production and preferably correspond to formula (II):R²O(CH₂CH₂O)_(n)H  (II)in which R² is a linear or branched alkyl and/or alkenyl groupcontaining 6 to 22 carbon atoms and n is an integer of 1 to 50.Compounds of formula (II) with a degree of ethoxylation of 1 to 20 areused for the synthesis in the process according to the invention for theproduction of hydroxycarboxylic acid esters. Typical examples are theadducts of on average 1 to 20, preferably 1 to 10 and more particularly1 to 5 mol ethylene oxide with caproic alcohol, caprylic alcohol,2-ethylhexyl alcohol, capric alcohol, lauryl alcohol, isotridecylalcohol, myristyl alcohol, cetyl alcohol, palmitoleyl alcohol, stearylalcohol, isostearyl alcohol, oleyl alcohol, elaidyl alcohol,petroselinyl alcohol, arachyl alcohol, gadoleyl alcohol, behenylalcohol, erucyl alcohol and brassidyl alcohol and the technical mixturesthereof obtained, for example, in the high-pressure hydrogenation oftechnical methyl esters based on fats and oils or aldehydes fromRoelen's oxo synthesis and as monomer fraction in the dimerization ofunsaturated fatty alcohols. Adducts of 1 to 10 mol ethylene oxide withtechnical C₁₂₋₁₈ fatty alcohols such as, for example, coconut oil, palmoil, palm kernel oil or tallow fatty alcohol are preferred.Alkyl and/or Alkenyl Oligoglycosides

Alkyl and alkenyl oligoglycosides are known nonionic surfactantscorresponding to formula (IV):R³O—[G]_(p)  (IV)where R³ is an alkyl and/or alkenyl group containing 4 to 22 carbonatoms, G is a sugar unit containing 5 or 6 carbon atoms and p is anumber of 1 to 10. They may be obtained by the relevant methods ofpreparative organic chemistry. The alkyl and/or alkenyl oligoglycosidesmay be derived from aldoses or ketoses containing 5 or 6 carbon atoms,preferably glucose. Accordingly, the preferred alkyl and/or alkenyloligoglycosides are alkyl and/or alkenyl oligoglucosides. The index p ingeneral formula (IV) indicates the degree of oligomerization (DP), i.e.the distribution of mono- and oligoglycosides, and is a number of 1 to10. Whereas p in a given compound must always be an integer and, aboveall, may assume a value of 1 to 6, the value p for a certain alkyloligoglycoside is an analytically determined calculated quantity whichis generally a broken number. Alkyl and/or alkenyl oligoglycosideshaving an average degree of oligomerization p of 1.1 to 3.0 arepreferably used. Alkyl and/or alkenyl oligoglycosides having a degree ofoligomerization of less than 1.7 and, more particularly, between 1.2 and1.4 are preferred from the applicational point of view. The alkyl oralkenyl radical R³ may be derived from primary alcohols containing 4 to11 and preferably 8 to 10 carbon atoms. Typical examples are butanol,caproic alcohol, caprylic alcohol, capric alcohol and undecyl alcoholand the technical mixtures thereof obtained, for example, in thehydrogenation of technical fatty acid methyl esters or in thehydrogenation of aldehydes from Roelen's oxosynthesis. Alkyloligoglucosides having a chain length of C₈ to C₁₀ (DP=1 to 3), whichare obtained as first runnings in the separation of technical C₈₋₁₈coconut oil fatty alcohol by distillation and which may contain lessthan 6% by weight of C₁₂ alcohol as an impurity, and also alkyloligoglucosides based on technical C_(9/11) oxoalcohols (DP=1 to 3) arepreferred. In addition, the alkyl or alkenyl radical R³ may also bederived from primary alcohols containing 12 to 22 and preferably 12 to14 carbon atoms. Typical examples are lauryl alcohol, myristyl alcohol,cetyl alcohol, palmitoleyl alcohol, stearyl alcohol, isostearyl alcohol,oleyl alcohol, elaidyl alcohol, petroselinyl alcohol, arachyl alcohol,gadoleyl alcohol, behenyl alcohol, erucyl alcohol, brassidyl alcohol andtechnical mixtures thereof which may be obtained as described above.Alkyl oligoglucosides based on hydrogenated C_(12/14) coconut oil fattyalcohol having a DP of 1 to 3 are preferred.Alkyl Ether Sulfates

Alkyl ether sulfates (“ether sulfates”) are known anionic surfactantswhich, on an industrial scale, are produced by SO₃ or chlorosulfonicacid (CSA) sulfation of fatty alcohol or oxoalcohol polyglycol ethersand subsequent neutralization. Ether sulfates suitable for use inaccordance with the invention correspond to formula (V):R⁴O—(CH₂CH₂O)_(m)SO₃X  (V)in which R⁴ is a linear or branched alkyl and/or alkenyl groupcontaining 6 to 22 carbon atoms, m is a number of 1 to 10 and X is analkali metal and/or alkaline earth metal, ammonium, alkylammonium,alkanolammonium or glucammonium. Typical examples are the sulfates ofaddition products of on average 1 to 10 and more particularly 2 to 5 molethylene oxide onto caproic alcohol, caprylic alcohol, 2-ethylhexylalcohol, capric alcohol, lauryl alcohol, isotridecyl alcohol, myristylalcohol, cetyl alcohol, palmitoleyl alcohol, stearyl alcohol, isostearylalcohol, oleyl alcohol, elaidyl alcohol, petroselinyl alcohol, arachylalcohol, gadoleyl alcohol, behenyl alcohol, erucyl alcohol and brassidylalcohol and technical mixtures thereof in the form of their sodiumand/or magnesium salts. The ether sulfates may have both a conventionalhomolog distribution and a narrow homolog distribution. It isparticularly preferred to use ether sulfates based on adducts of onaverage 2 to 3 mol ethylene oxide with technical C_(12/14) or C_(12/18)coconut fatty alcohol fractions in the form of their sodium and/ormagnesium salts.

The surfactant mixtures according to the invention may be used for theproduction of cosmetic preparations such as, for example, hair shampoos,hair lotions, foam baths, shower baths, creams, gels, lotions, alcoholicand aqueous/alcoholic solutions, emulsions, wax/fat compounds, stickpreparations, powders or ointments. The hydroxycarboxylic acid estersaccording to the invention may also be used in combination with otherauxiliaries and additives typically used for cosmetic preparations suchas, for example, mild surfactants, oil components, emulsifiers,superfatting agents, pearlizing waxes, consistency factors, thickeners,polymers, silicone compounds, fats, waxes, lecithins, phospholipids,stabilizers, biogenic agents, deodorants, antiperspirants, antidandruffagents, film formers, swelling agents, UV protection factors and thelike. Another embodiment of the present invention is the use ofhydroxycarboxylic acid esters produced by the process claimed in claim 1as auxiliaries for the production of cosmetic preparations, preferablythose containing either alkyl ether sulfates or alkyl oligoglycosides orfatty alcohol ethoxylates or mixtures of these surfactants.

It can be of advantage to use mixtures of APG compounds corresponding toformula (IV) with the hydroxycarboxylic acid esters according to theinvention for the production of cosmetic preparations in which the ratioby weight of the APGs to the hydroxycarboxylic acid esters or mixturesthereof is in the range from 3:1 to 1:3. Aqueous formulations areparticularly preferred, particularly if they are mildly acidic andpreferably have a pH of5to 6.5.

EXAMPLES

1. Preparation of a Monoalkylcitric Acid

299.6 g (1.6 mol) of a C₁₂ fatty alcohol were introduced into a stirredvessel with a water separator together with 127.2 g (0.4 mol) of aC₁₂₋₁₄ fatty alcohol, which had been reacted with 3 mol ethylene oxideper mol fatty alcohol, and 384.2 g (2.0 mol) of water-free citric acidand heated under nitrogen to 160° C. On completion of the reaction, thereaction mixture was cooled to 100° C. and the end product was obtainedby distillation. A yellow, viscous bright ester was obtained (yield775.0 g).

The saponification number was 432, the acid value measured 284, the OHvalue was 210, the ester value was 148. The percentage content of freecitric acid was 3.7% by weight.

2. Performance Tests

A mixture of an ether sulfate (Texapon N, a product of CognisDeutschland GmbH & Co. KG) both with pure lauryl citrates (C1) and withlauryl-/C₁₂₋₁₄ fatty alcohol+3EO citrate in a ratio by weight of 3:1 or9:1 (E1 and E4) (based on active substance) in water was prepared, thepH of the solution being adjusted to 6.

Results: C1 Lauryl citrate Cloudy, separating liquid 2 Lauryl-C₁₂₋₁₄fatty alcohol + Clear, bright liquid 3EO citrate (9:1) C3 Laurylcitrate + C₁₂₋₁₄ fatty Cloudy, separating liquid alcohol + 3EO citrate(9:1) 4 Lauryl-C₁₂₋₁₄-fatty alcohol + Clear, bright liquid 3EO citrate(3:1)

It is clear that only the products according to the invention lead touseful products; even the subsequent mixing of alkyl and alkyloxycitrates (C3) does not produce the required result.

In addition, a commercially available alkyl (oligo)glycoside (APG),Plantacare® 1200 of Cognis Deutschland GmbH & Co. KG, was made up intoan aqueous formulation with various alkyl citrates (pH 6). The ratio byweight of APG to alkyl citrate was 3:1. The total active substancecontent of the solutions was 30%.

Results: Lauryl citrate Inhomogeneous cloudy dispersionLauryl-C₁₂₋₁₄-fatty alcohol + Homogeneous, slightly opaque solution 3EOcitrate (9:1) Lauryl-C₁₂₋₁₄-fatty alcohol + Homogeneous, slightly opaquesolution 3EO citrate (3:1)

1-10. (canceled)
 11. A process for the production of a mixture of mono-,di- and triesters of hydroxycarboxylic acids, said process comprisingreacting hydroxycarboxylic acids or salts thereof with a mixture ofalcohols corresponding to the formulas (I) R¹—OH and (II)R²—(C₂H₄)_(n)—OH, where R¹ and R² independently represent a saturated orunsaturated, branched or unbranched C₆₋₂₂ alkyl group and n is a numberof 1 to 20, wherein the compounds of formulas (I) and (II) are used in aratio by weight of 10:1 to 1:1 and the hydroxycarboxylic acid is used insuch quantities that the molar ratio of the free carboxyl groups of thehydroxycarboxylic acid to the free OH groups of the alcohols is at most3.5:1.
 12. A process according to claim 11, wherein the alcohols offormulas (I) and (II) are used in a ratio by weight of 4:1 to 1:1.
 13. Aprocess according to claim 11, wherein the alcohols of formulas (I) and(II) are used in a ratio by weight of 1:1.
 14. A process according toclaim 11, wherein the hydroxycarboxylic acid is used in such quantitiesthat the molar ratio of the free carboxyl groups of thehydroxycarboxylic acid to the free OH groups of the alcohols is at most3:1.
 15. A process according to claim 11, wherein the reaction iscarried out at temperatures of 150 to 170° C.
 16. A process according toclaim 11, wherein the hydroxycarboxylic acid is selected from the groupconsisting of lactic acid, tartaric acid, malic acid, citric acid andself-condensation products thereof.
 17. A process according to claim 16,wherein the hydroxycarboxylic acid is citric acid.
 18. A processaccording to claim 11, wherein the resulting mixture ofhydroxycarboxylic acid esters contains 25 to 30% of monoesters, 30 to40% of diesters, 5 to 15% by triesters, and up to 8% of freehydroxycarboxylic acid.
 19. A mixture of mono-, di- and triesters ofhydroxycarboxylic acids, and optionally free, hydroxycarboxylic acid,the mixture comprising esters corresponding to the formula (III):

in which R′, R″, R′″ represent a hydrogen atom, a C₆₋₂₂ alkyl group, oran ethoxylated C₆₋₂₂ alkyl group, the ethoxylated alkyl groupscontaining 2 to 20 parts ethylene oxide per alkyl group, with theprovisos that at least one of the substituents R′, R″ and R′″ representssuch an ethoxylated alkyl group, and wherein the ratio of mono- todiesters in the mixture is 1:1 to 1:2.
 20. A mixture according to claim19, wherein the ethoxylated alkyl groups contain 2 to 10 parts ethyleneoxide per alkyl group.
 21. A mixture according to claim 19, wherein thehydroxycarboxylic acid is selected from the group consisting of lacticacid, tartaric acid, malic acid, citric acid and self-condensationproducts thereof.
 22. A mixture according to claim 19, wherein thehydroxycarboxylic acid is citric acid.
 23. A mixture according to claim19, wherein the ethoxylated alkyl groups contain 2 to 10 parts ethyleneoxide per alkyl group and the hydroxycarboxylic acid is citric acid. 24.A mixture according to claim 19, consisting of 25 to 30% of monoesters,30 to 40% of diesters, 5 to 15% by triesters and up to 8% of freehydroxycarboxylic acid.
 25. A cosmetic composition comprising (A) one ormore cosmetically active ingredients and (B) a mixture of mono-, di- andtriesters of hydroxycarboxylic acids, and optionally free,hydroxycarboxylic acid, the mixtures containing esters corresponding tothe formula (III):

in which R′, R″, R′″ represent a hydrogen atom, a C₆₋₂₂ alkyl group, oran ethoxylated C₆₋₂₂ alkyl group, the ethoxylated alkyl groupscontaining 2 to 20 parts ethylene oxide per alkyl group, with theprovisos that at least one of the substituents R′, R″ and R′″ representssuch an ethoxylated alkyl group, and wherein the ratio of mono- todiesters in the mixture is 1:1 to 1:2.
 26. A cosmetic compositionaccording to claim 25, wherein the ethoxylated alkyl groups of component(B) contain 2 to 10 parts ethylene oxide per alkyl group.
 27. A cosmeticcomposition according to claim 25, wherein the hydroxycarboxylic acid ofcomponent (B) is citric acid.
 28. A cosmetic composition according toclaim 25, wherein, in component (B), the ethoxylated alkyl groupscontaining 2 to 10 parts ethylene oxide per alkyl group and thehydroxycarboxylic acid is citric acid.
 29. A cosmetic compositionaccording to claim 25, the mixture of component (B) consists of 25 to30% of monoesters, 30 to 40% of diesters, 5 to 15% by triesters and upto 8% of free hydroxycarboxylic acid.
 30. A cosmetic compositionaccording to claim 25, further comprising alkyl ether sulfates, alkyloligoglycosides or fatty alcohol ethoxylates or mixtures thereof.